The present invention relates to a force sensor of the type using an electric resistance that varies according to the applied force.
Force or pressure sensors whose electric resistance varies with the applied force are known as FSRs (force sensing resistors) and make possible a direct determination of the force applied to the active surface of the sensor.
Such sensors are, for example, described in the documents EP 0 649 518 and U.S. Pat. No. 4,314,227. They comprise two flexible supporting sheets of insulating material which are arranged opposite each other and separated by a certain distance by means of an intercalated separator. The separator comprises, for example, a two-sided adhesive band which is cut in such a way as to surround at least partly the active zone of the sensor. Inside the active zone, one of the supporting sheets is provided with two electrode structures made of a conducting material and separated from each other, while the other supporting sheet is provided with a coating of a pressure-sensitive semiconducting or resistive material. The semiconducting or resistive material may either have an internal resistance that decreases when the layer is compressed, or may have microprojections on the surface so that the surface resistance between the layer and a conductor decreases when the layer is pressed on to the conductor.
When no pressure is acting on the pressure sensor, the layer of semiconducting material is not in contact with the two electrodes and the electric resistance between the two electrodes is consequently very high. If, on the contrary, a pressure is applied to the sensor, the two supporting sheets are pressed together and the pressure-sensitive layer is put into contact with the two electrodes. This produces, between the two electrodes, a short circuit whose electric resistance varies inversely with the value of the applied pressure. The greater the pressure on the sensor, the more the semiconducting layer is compressed or the more it comes into intimate contact with the electrodes, and the more the resistance measured between the two electrodes decreases.
In a mode of execution of the sensor according to the document U.S. Pat. No. 4,314,227, each electrode comprises fingers extending from a main conducting structure, said fingers of the two electrodes being arranged so that they mesh with each other. Both electrodes are coated with a layer of semiconducting material, while the other supporting sheet is provided with a coating of conducting material. Such a sensor has a very good dynamic range due to the many points of contact between the coating of conducting material and the layer of pressure-sensitive material on the fingers of the two electrodes. The alternating arrangement of the fingers of the two electrodes, on the other hand, causes problems related to the tolerances in production. In fact, because of the small distance between two neighbouring fingers, variations in the production of this type of sensor often bring about short circuits between the two electrodes, thus making the sensors unusable.
It follows that the tolerances allowed in the production of these sensors are very restrictive, which makes the production slow and costly.
The objective of the present invention is to propose a force sensor which is less sensitive to variations in production.
This objective is attained by a force sensor comprising two electrodes made of a conducting material, which are arranged so that they are separated from each other on a first insulating support, each of said two electrodes being coated with a layer of pressure-sensitive material, the two layers of pressure-sensitive material being electrically insulated from each other, and a contact element made of a conducting material is arranged at a certain distance from the two electrodes, said contact element being pressed against said electrodes when a force is exerted on the force sensor. In conformity with the invention, each electrode comprises a conductor arranged substantially on the periphery of an active zone of the sensor and the layer of pressure-sensitive material covering each electrode extends towards the interior of the active zone, the two pressure-sensitive layers being separated by an interstice that passes substantially through the center of the active surface.
Although such a sensor has a dynamic range slightly smaller than that of sensors known in the present state of the art, its dynamic range is good enough for many applications where such a good dynamic range is not essential. On the other hand, because of the well-separated arrangement of the electrodes, short circuits between the electrodes caused by variations in production are significantly reduced in comparison with those in sensors of the present state of the art. The only short circuits that may be produced are those between the pressure-sensitive layers. However, because of the very high specific resistance of these short circuits between the pressure-sensitive layers, with a value above the operational threshold of the sensor, any possible short circuits do not interfere with the proper functioning of the sensor.
Consequently, the production tolerances for such a sensor may be less strict, which allows production to be faster. Moreover, the production of sensors not conforming to the specifications is significantly reduced, which increases the productivity of the production line.
In addition to the aforesaid advantages, the use of the pressure-sensitive layer to cover the electrodes protects the surfaces of said electrodes against contact with air. This eliminates a serious problem posed by the use of electrodes that are slowly oxidised when they are exposed to air.
The layer of pressure-sensitive material may comprise either microprojections on the surface, so that the surface resistance between the layer and the contact element decreases with the pressure exerted on the junction between the layer and the contact element, or a material whose specific resistance varies inversely with the compression of said material, or a combination of the two. It may for example involve a semiconducting polymer or a conducting elastomer.
In order to adapt the dynamic range and sensitivity of such a sensor to the specific requirements of an application, the layer of pressure-sensitive material preferably comprises inclusions of conducting material, said inclusions of conducting material being arranged in such a way as to change the specific resistance of the layer of pressure-sensitive material. This forms a very significant advantage if one wishes to manufacture several sensors with different sensitivities on the same support, particularly for the manufacture of seat-occupancy detectors, which comprise several force sensors arranged alongside each other on a sheet. Such a manufacture then becomes possible without having to use different pressure-sensitive materials for the different sensitivities. It is sufficient to vary the number and the placing of the inclusions adequately in order to achieve the desired sensitivity.
The sensitivity of each force sensor can therefore be adjusted by a modification of the geometrical arrangement of its various components. In this way, the sensitivity can be adapted over a wide range to all requirements and, in particular, it becomes easily reproducible, i.e. it would be very easy to provide an exact reproduction of a required sensitivity. Moreover, the sensitivity of the cell becomes largely independent of the thickness of the layer of pressure-sensitive material, which makes possible a further increase in the speed of manufacture of the sensors.
The contact element of conducting material preferably comprises a layer of conducting material, graphite for example, applied to a second flexible support. The second support is then placed at a distance from said first support by means of a separator located outside an active zone, so that in the interior of said active zone the layer of conducting material is opposite said electrodes. Said separator advantageously comprises a printable adhesive which serves to stick said first substrate to said second substrate. The adhesive may, for example, be applied by serigraphy, just like the electrodes and the layers of pressure-sensitive material, or by spray printing. After printing the adhesive and assembling the two supports, the adhesive is hardened, for example, by baking.
The use of a printable adhesive allows great freedom in the design of force sensors, particularly during the manufacture of a set of sensors on one substrate, like seat-occupancy detectors. In fact, until now, a two-sided adhesive band has generally been used as a separator. This adhesive band was cut before assembly, so that it had cut-out sections in the shape of active zones at places which, after assembly of the sensor, corresponded to its active zones. In order to provide, in the assembled sensor, ventilation chambers connecting the active zones with the environment and thus allowing an equalisation of pressure between the active zones and the environment, these cut-out sections must partly be connected by thin cut-out sections. This practice clearly results in quite strict limitations on the design of detectors since, after application of the cut-out sections of the separator, the latter must still form a single piece in order that it can be manipulated during the assembly of the detector. Moreover, its shape must be adapted so that a protective sheet can easily be removed from the adhesive surfaces.
All such limitations on the shape of the separator no longer occur when the latter is simply printed by a suitable technique on a substrate or substrates before their assembly.
In order to ensure a uniform spacing of the two supports over the whole extent of the sensor, separator particles with a diameter substantially equal to the desired spacing of the two substrates are preferably arranged inside said printable adhesive. These separator particles may be mixed in the liquid adhesive and applied together with it, or they may be introduced into the adhesive after it has been applied.
The force sensor as described above is thus particularly well suited to the manufacture of seat-occupancy detectors, comprising one or more force sensors. It allows a fast and highly productive production of such occupancy detectors while minimising production losses. Since the manufacturing process has largely eliminated constraints on the design of the detectors, it could easily be adapted to minimise the losses of substrate by offcuts, for example.